Electrical communication system



May 24, 1949. P. CURRY ELECTRICAL COMMUNICATON SYSTEM 5 Shee'ts-Sheet l NOV. 2J 1946 Emmi um@ lill l W M May 24, 1949. P. cuRRY ELECTRICAL COMMUNICATION SYSTEM 5 Sheets- Sheet 2 Filed Nov. 2, 1946 ATTORNEYS.

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ELECTRICAL COMMUNICATION SYSTEM Filed Nov. 2, 1946 5 Sheets-Sheet 4 Admw QN @QM -IWANN -f T. m m En n mm M m .mm M m mp W I, aum 1 ,1mmN l-l \I4-| \.Im1wl 23-45. Mw

5 Shea-:dis-Sheel 5 Filed Nov. 2, 1946 h -CYCLE 0F UPPLEMENTAL MonuLA-nou 516ML) l CYc'LE oF SUPPLEMEM'AL Mo vuLAnoN (lcuAL) `/A/vENTo/e CYcLE oF SUPPLENTAL HeouLATwN (-5\cvN/4L) 1' PAU/- CURE Y APPuEvAs AoomouaL FuouENcY-Hoouunon BIJ 2 'Li A rTo/e/vfrs Patented May 24, 19419 ELECTRICAL COMMUNICATION SYSTEM Paul Curry, New Haven, Conn. Application November 2, 1946, Serial No. 707,348

12 Claims.

The present invention relates to systems whereby speech, music, or other intelligence may be communicated to and received at another location. Such electrical communication-systems may be either via wire or may be wireless in character.

The present application constitutes a continuation-in-part of my copending application Ser. No. 524,545, filed March 1, 1944, now abandoned.

One of the objects of the present invention is to provide a superior communication-system whereby the reception of intelligence by unauthorized persons is made exceedingly diflicult if not impossible.

Another object of the present invention is to provide a superior communication-system of the character referred to which may be utilized either for wireless or wired communications.

A further object of the present invention is to provide a superior electrical communication-system of such character that a maximum number of communication channels becomes available without interference.

With the above and other objects in view, as will appear to those skilled in the art from the present disclosure, this invention includes all features in the said disclosure which are novel over the prior art and which are not claimed in any separate application.

In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes;

Fig. 1 is a diagrammatic view of one form of wireless sendingor transmitting-apparatus suitable ior carrying out the present invention and designed to transmit two carrier-waves both of which are frequency-modulated and amplitudemodulated;

Fig. 2 is a similar view of a wireless receiving apparatus suitable -for receiving the doublymodulated carrier-waves of the sending-apparatus of Fig. 1, or Fig. 4;

Fig. 3 is a view schematically illustrating one example of the frequency-modulation characteristics which the two doubly-modulated transmitted carrier-waves may have, together with the frequency-modulation characteristics of a wave which maybe generated at the receiving-station;

Fig. 4 is a view similar to Fis. 1, but illustrating another form of sendingor transmitting-apparatus designated to transmit two carrier-waves both of Vwhich are doubly-frequency-modulated;

Fig. 5 is a schematic representation of a carrier-wave having basic-frequency-modulation;

Fig. 6 is a similar view of a signal-wave suit- 2 able for additionally modulating the carrier-wave shown in Fig. 5;

Fig. 7 is a schematic representation of the frequency-modulated carrier-wave of Fig. 5, as additionally amplitude-modulated by the signalwave of Fig. 6; and

Fig. 8 is a schematic representation of the frequency-modulated carrier-Wave of Fis. 5, as additionally frequency-modulated by the signalwave of Fig. 6.

The particular electrical communicating-apparatus or -system herein chosen for purposes oi.' illustrating one form of the present invention, is primarily designed for wireless communication, though, as will hereinafter appear, is equally suitable for use in connection with a system in which conductors connect a transmitting-station and a receiving-station.

THE APPARATUS OF FIGS. 1 AND 2 The apparatus illustrated includes a transmitting-antenna Illab and a receiving-antenna I lab. In order to aiord a better underdstanding of the apparatus which will be more fully hereinafter described, it may be here stated that the transmitting-antenna Inab coincidentally sends out to the receiving-antenna il ab, two separate Irequency modulated electro-magnetic carrierwaves. Each of the said carrier-waves has both its upper frequency-limit and its lower frequencylimit diil'ering from the similar limit of the other carrier-wave, and both of the said carrier-waves being further modulated (amplitude-modulated, in the instance shown) by imposing the signal to be transmitted on both of the said can'ier-waves, but in an out-of-phase relationship respectively. The two doubly-modulated carrier-waves picked up by the receiving-antenna iiab are handled in a manner as will be later described under the heading Receiving system. The transmittingsystem or -apparatus will be first described.

THE TRANSMITIDIG SYSTEM acier with a subscript a. Similarly. such elements as are involved primarily only with the wave B will bear reference characters followed by the subscript b."

ulator |8a is yconnected to a signal-input unit designated by the reference character Iab, and which is also used in connection with the wave B, vas will later appear.

The wave-generator |20. above referred to, receives a frequency-modulating wave from a frequency-modulator Zliab which, incidentally, is also used in connection with the wave B previously referred to, all as will more fully hereinafter appear. The output of the frequency-modulator ab just referred to, is connected to an amplifying-unit 2Iab which, like the frequency-modulator 20ab, is also used in commotion with the wave B. The output of the amplifying-unit Zlab is connected to an amplifying-unit 22a, which latter in turn has its output connected to the wave-generator Iza, before referred to.

Preferably and as shown, the input-side of the' frequency-modulator 20ab above described, is connected to an amplitude-limiting unit 23u17 in such manner as to impose a limit on the amplitude of the wave produced by the frequencymodulator 20ab.

The various wires interconnecting the features above described do not require detailed description, other than to note that the input of the amplitude-limiting unit 23ab is connected to the output of the amplitude-'modulator |8a.

The output of the amplitude-modulator 48a above described, is connected by a coupling-transformer 24a to the transmitting-antenna lab.

The amplitude-limiting unit 23ab includes a primary inductance-coil 25ab which is connected through a condenser 26u17 to the output of the amplitude-modulator I8a. The primary inductance-coil 25ab cooperates with two secondary inductance-coils Z'lab and 28ab forming features of the unit 23u17. Each of the secondary induction-coils lab and 28ab has respectively connected across it one of two variable condensers 29u21 and 30ab, each of which acts in conjunction with its complemental inductance-coil to provide a resonant circuit.

The tuned circuit in which the secondary inductance-coil 21ab and the condenser 29ab are included, is resonant to a frequency widely diierent from the resonant-frequency of the circuit in Winch the coil 28ab and condenser 30ab are included. Therefore, the primary inductance-coil 25ab is -so constructed as to pass the two frequencies to which the circuits of the secondary coils .Hab and 28ab are respectively responsive.

The circuit of the elements 21ab-2 Sab includes one of the plates of a twin diode electronic tube 3Iab, while the circuit in which the elements 28ab-30ab are located is connected to the other of the plates of the said tube.

It may here be noted that the resonant circuit in which the elements Nah-29u11 are included,

develops a voltage which is opposite to the voltv age developed by the energy in the lresonant cir t of which the elements nab-30a?) form a p These opposed voltages appear alternately at the output of the amplitude-limiting unit 23ab and are applied to the grid of aY gas-filled triode 32ab forming a feature of the frequency-modulator "ab before referred to.

In addition to the gas-filled triode 32ab, the

frequency-modulator Zllab also includes suitabler resistors and a variable condenser as shown, to thus produce an output-wave having saw-tooth or logarithmic characteristics.

As before noted, the output of the frequencymodulator 20ab is connected to the input of the amplifying-unit Zlab. The amplifying-unit Zlab is used in conjunction with the production of both of the waves A and B before referred to, and includes a pentode electronic tube generally designated by thereference character 33ab. It is the control-grid of the tube 33ab which is connected through a suitable resistor and variable condenser to the output of the frequency-modulator 20ab. The output-lead of the amplifying-unit 2|ab is electrically connected to the amplifyingunit 22a before referred to, as well as to a second amplifying-unit 22h which will be later referred to The amplifying-unit 22a just referred to serves now only in connection with the production of the wave A before referred to and includes a triode electronic tube generally designated by the reference character 34a. The adjuncts of the triode 34a just referred to are clearly shown in Fig. l and do not require detailed description other than to note that the cathode of the tube 34a constitutes the output of thefarnplifying-unitia and is connected to the input of the wave-generator |2a before referred to.

The wave-generator IZa includes a triode electronic tube generally designated by the reference character 35a, which is here utilized as a superregenerative detector or blocking-oscillator tube in a manner well understood in the art. The adjuncts of the tube 35a are schematically represented in Fig. 1 and do not require detailed description other than to possibly mention that it is the cathode of the tube 35a which is connected to the output of the amplifying-unit 22a. The plate-circuit of the tube 35a is connected to the high-pass filter 43a before referred to, which latter, in turn, is connected to the input of the amplifying-unit Ma.

The amplifying-unit Ita includes a pentode electronic tube 36a having its control-grid circuit connected to the output of the high-pass filter l3a,`as is indicated in Fig. 1. Here again, the adjuncts of the amplifying-unit Ma do not require detailed description other than to note that the plate-circuit of the tube. 36a is made resonant by the inclusion of a condenser 31a and a, primary inductance-coil 38a.

The primary inductance-coil 38a of the amplifying unit lla cooperates with a secondary inductance-coil 39a constituting the input of the combined rectifying-and-amplifying unit I5a. The coil 39a just referred to, in conjunction with a condenser 40a, provides a resonant circuit connected across the diode section of a combined diode-triode electronic tube generally designated by the reference character lla. The output of the resonant circuit which includes the elements 39a and 40a, is connected to the grid of the triodesection of the tube lla, while the load-resistor 82a and associated condenser 83a ofv the diodelection are connected to the cathode of the tube Ila through a biasing-battery 41a, which latter serves to impose a bias-voltage on the grid of the triode-section of the combination tube lia, as is clearly shown in Fig. 1. The plate of the triodesection of the tube lia includes a primary inductance-coil 43a.

The primary inductance-coil a of the unit lia cooperates with a secondary inductance-coil a forming a feature of the amplifying-unit IIa. One terminal of the secondary inductance-coil a is connected to the control-grid of a pentode electronic tube lla forming a feature of the amplifying-unit ila. The adjuncts of the tube lla are clearly illustrated in Fig. 1 of the drawings and do not require detailed description other than to notev that the plate ofthe tube a is connected to the input of the band-pass filter IIa before referred to.

The output of the band-pass filter i'la is connected to a coupling-circuit which includes 'a coupling-condenser a and an inductance-coil lla, forming, in the instance shown, features of the amplitude-modulator lia before referred to. The circuit which includes the elements a and "a, is so constituted as to pass the band-width of frequencies which is the output of the filter ila. The circuit of the elements a and 1a is connected to the control-grid of a pentode electronictube "a, also included as a feature of the amplitude-modulator lla.

The features of the amplitude-modulator Ila do not require further detailed description other than to note that the plate of the tube 48a is co nected to the primary of the antenna-coupling transformer 24a before referred to. Further note may be had of the fact that the suppressor-grid of the tube 48a is connected to one section of the secondary coil of lthe transformer of the signalinput unit I'Sab before referred to.

TH: APPARATUS ron PuoDUcING 'rHl DOUBLY- Monuuran Wav: B

'and 2lb and respectively corresponding to the units |2a to i'a inclusive and 22a and 24a.

The elements previously described in connection with the various instrumentalities [2a to lila i inclusive and 22a and 24a, are duplicated in the similar elements which are designated by the subscript b and bear like reference characters save that they bear the subscript b instead of the subscript a.

yFrom the foregoing it will appear that the output of the amplifying-unit 2mb flows to both theamplifying-units 22a and 22h. The plate of the tube b of the amplitude-modulator IIb is connected to the primary on the antenna-coupling transformer 2lb. It should also be noted that the suppressorgrid of the tube b is connected to the section of the Secondary of the signal-input unit lQab opposite to that to which the suppressor-grid of the tube "a is connected. Thus, the voltage applied to the suppressor-grid of the tube a will be in phase-opposition to the voltage applied to the suppressor-grid of the tube 8b.

In conjunction with the transmitting-apparatus above described, three batteries or other suitable sources of electrical energy "ab, "ab

8 and llab are employed. In the particular instance shown, the battery abl serves to supply the heaters of the various electronic tubes employed in the transmitting system, while'the battery Slab vis suitably connected to such of the various electronic tubes as include a screen-grid to impress a suitable intermediate voltage on such screen-grids. The battery'iab in conjunction with the battery "ab, is connected to the respective plates of the various electronic tubes of the transmitting-system with the exception of the twin diode-plates of the tubes Il, Ma and lib.

Tm: Evourrron or A Trrrcu. Wav: A

, For purposes of clarity of description, let it be assumed that the wave A with respect to its frequency -modulatedl characteristics, has a band-width having a lower lfrequency-limit of 900 kilocycles and an upper frequency-limit ot 1800 kilocycles.v

Under the circumstances above outlined, the wave-generator ia, which is a self-blocking super-regenerative detector or oscillator will be constructed and arranged to produce a funda-v mental-wave having a fundamental-frequency of 50 kilocycles, together with the harmonic-frequencies thereof, which latter have pronounced values. This fundamental-frequency wilLhow ever, be frequency-modulated by the frequency.-

of waves each corresponding to the others both in pattern and in time-interval and all of which groups occur between an upper frequency-limit of 50 kilocycles and a lower frequency-limit of 35.25 kilocycles. The resultant frequencies between the limits just referred to, represent the rates of quenching or suppression of the wave-genera tor I2a. The harmonic-frequencies of the aforesaid fundamental-frequency will also be frequency-modulated by the frequency-modulator lllab.

It may here be noted that the output of the frequency-modulator Zilab, before its application to the wave-generator i2a, is amplified by the :mplifying-unit Zlab and the amplifying-unit Under the circumstances above described, the frequency-modulator 2Mb will have an outputfrequency of 10 kilocycles, in order to jointly with the wave-generator I2a, contribute to the frequency-modulated characteristics of the wave A. Furthermore, the output of the frequency-modulator 20ab is characterized by a logarithmic or sawtooth wave-form (rather than a sinusoidal wave-form) having the time of its rise corresponding to the time of its decline. While the output-wave of the frequency-modulator 2Mb is logarithmic in character as just noted, nevertheless, such logarithmic wave-form 'is kept as near to linear rise and decline respectively, as is feasible. Such linear characteristics enable the present system to operate with maximum facility, though they are not essential.

At this point, it is well to here note the function of the amplitude-limiting unit 23ab. This unit Nah serves to limit the amplitude of the output of the frequency-modulator 20ab, in order to hold the frequency-limits of the wave A (as well as the wave B) at predetermined values. Thus, under the conditions here being described, the resonant network comprising the elements 21ab and ab will preferably be broadly tuned to a band between about 900 kc. and 1000 kc., while the network comprising the elements ab and modulator 2Mb so as to result in recurrent groups anarco and the harmonic-frequencies thereof (all of.

which have been frequency-modulated) are applied to the input of the high-pass nlter Ila.

Under the particular circumstances now being considered, the characteristics of the high-pass filter lla will be such as to cause the maximum attenuation of all frequencies below about 7,650 kilocycles in value (inclusive of the aforesaid group of fundamental-waves) while causing theminimum attenuation of all of the frequencies at and above about 7,650 kilocycles. The net result will be that at the output of the high-pass filter ila, the frequencies at and above about 7,650 kilocycles in value will appear with suicient value to be employed in the subsequent steps of the system, whereas, the frequencies below about 7,650 kilocycles will have negligible values and will not affect the subsequent steps of the system.

As before noted, the amplifying-unit Ha has a resonant network which includes the elements 31a and 38a which, in conjunction with the resonant circuit (including the elements 39a and 40a) of the unit |511, will freely pass only the selected frequency which, in this instance, will be a-relatively narrow band centered at about 7,650 kilocycles. The frequency-band last referred to will, of course, have been amplified in the unit Ha. As is well known in the art, frequencies other than the selected frequency of about 7,650 kilocycles, will pass from the unit lla to the unit lia, butin such negligible amplitude as not to affect the following stages of the system.

With the particular relationship being described, the group of frequency-modulated harmonic-frequencies extending between and including the 153d to 217th will successively appear in the resonant network comprising the elements 37a and 38a o f the unit Ita and the elements 39a and 40a of the unit 45a. The said group of harmonic frequencies will, however, only pass through the said network at the instant that they respectively successively come into resonance therewith. As each of the aforesaid frequencymodulated harmonic-frequencies (sixty-four in number) successively appears in the resonant network just referred to, each will at some point in the cycle of its frequency-change, come into resonance (about 7,650 kilocycles) with the aforesaid resonant network, and will, while in substantial resonance. cause an increment of energy to appear in the said resonant network. Owing to their frequency-modulated character, the timeintervals between the respective maximum-values of the said increments of energy, will vary. This variation in time-intervals 'results from the fact that the different harmonic-frequencies of the aforesaid fundamental-wave, progressively vary arithmetically in time-interval. Thus, the timeinterval between the 153d and the 154th harmonies may, for convenience of illustration, be considered to be equal to 1/53, in which case the relative time-interval between the 154th and the 155th harmonics will be smaller and equal to 1/m and so on to the 216th and 217th harmonics which willhave 9, still smaller relative time-interval of 1/z1e. It is this arithmetical progression or relationship which determines the ultimate frequency-modulation characteristics of the wave A.

The radio-frequency components of each of the increments 'of energy Just above referred to. will. vary in amplitude and the said increments will occur in groups of sixty-four which groups provide what is commonly known in the art as a wave-envelope." y

The radio-frequency waves representing each of the aforesaid increments of energy are rectied by the diode-section of the tube Ila, with the result that half of each of the said waves and half of the envelope thereof, are eliminated. The remaining halves of the said waves are so ltered by the load-resistor 52aV and its complemental condenser 63a, that there appears (amplified) in the output of the triode section of the tube lla, a new frequency-modulated fundamental-wave having frequency-limits respectively at 900 kilocycles and 1800 kilocycles, together with a series of harmonic-waves representing harmonics of all the waves occurring between the said limits of 900 kilocycles and 1800 kilocycles inclusive.

It may be here shown, by example, through one cycle of operation, how the output of the unit 2Mb, in conjunction with the operation of blocking oscillator l2a and the tuned network comprising the elements 31a, 38a, 39a and a which is tuned to a relatively narrow band centered on 7650 kilocycles, as well as the rectifying network -in which the elements 39a and 40a are included.

output of unit 20gb is, as shown above, 10.0 kilocycles per second.

A further assumption, for purposes of illustration, is that the rising function of the output of the unit 20ab, which equals X. and the declining function which equals 1-X=X represent straight lines. This assumption is permissible since in practical cases the logarithmic rise and decline of a saw-tooth wave can be made to approach linearity sufficiently to support the illustration.

A still further valid assumption is that the frequency variation of the output of oscillator ila is linear with the rise and decline of the function of the unit 20a!) output, since the actual logarithmic variation of the frequency of the 12a oscillator ouput, with the variation of bias voltage across the tube 35a cathode resistor can be made to approach linearity sufficiently to support the illustration.

The fundamental frequency of 35.2535 kilocycles, at the output of oscillator Ila, has a large number of higher order harmonic frequencies harmonicallyspaced in frequency, so that the second harmonic has a frequency of 70.507 kilocycles, the third harmonic has a frequency of 105.7605 kilocycles, etc. It can be seen therefore, that the 217th harmonic has a frequency of, substantially, 7.650 kilocycles, and is, therefore, in

' resonance with the tuned network comprising the elements 31a, 38a, 39a and 40a. In consequence, the voltage developed across the element lla, which. together with its complemental condenser Bla, forms the load network of the diode substantially out of resonance with the tuned netv atmaunmmmaeonsoumtofumt 1s resistm'llalsainatmaximum 25 Itcaubeleentromthedeecription above,that from time h to time ta the voltage developed the diode-load-resistor lia has described afunctlonwhichadeclineforthe perlodfrumtimetototimetaandarisefrom timetntotimeta. Andthedeelinetogetherwith theriee. awave functionwhich canbe mathematically resolved into a fundamental sine wave. plusalsrlenumberofsine wave harmonics ofthesaidflmdamentalsinewave.

time from a to t., vane the nmcuon of the out- 4o putofunitabisrisingfromserotcmaidmum,

l and from t. to t+n. while the function is declining from maximum to aero againrthis representing onecycle in the operation of the unit floh.

a frequency of '1.6313333 kilocycles it, too. is 10 Since, as has been noted. the frequency of the output of unit ab is 10.0 kilocycles per second, and the time of the rising function of the ab output represents one-half cycle of operation, the timefromtototnislhmsecond.

And. since the output of oscillator lia is 35.2535 kilocycles at time to, and 50.0 kllocycles at time tu, the variation in frequency in $60,000 second is 14.7465 kilocycles.

The variation in frequency of the oscillator Ila output from time te to time tz is 35.4166 minus 35.2535 equals .1631 kilocycle and, since the frequency variation dllrinl 150.000 second is 14.7465 kilocyeles the time interval represented by tn-tz is .1631/1411465 X .00005 equals .000000553 second.

It can be seen. therefore. that if the function of the wave developed by the voltage across the diode-load-resistor lla during the time to-tr were recurrent, the frequency of that function would be 1808.3 kilocycles. 'Ihis frequency is substantially near one frequency limit (1800l kc.) of the fave A, as shown in this disclosure.

To determine the frequency of the voltage wave-function developed across the load-resistor 52a, during the time interval tz-ti we have a frequency dinerentiation in the oscillator lia output of .1648 kilocycle, and, since the frequency variation during 0,000 second is 14.7465 kilocycles the time interval represented by tz-t4 is .1648/143465 x .00005 equals .00000055875 second.

It is seen, therefore, that if the wave-function developed by the voltage across the diode-loadresistor 52a during the time interval iba-t4 were recurrent, the frequency of the wave-function would be 1,789.7 kilocycles. 'I'he frequency of that wave-function during theinterval tz-t4 is there-,

fore 16.6 kilocycles lower than the wave-flmction during the previous interval ta-tz.

It may not be necessary to show, individually, the 64 successive voltage wave-functions appearing across the diode-load-resistor 52a during the successive time intervals ti-te. ts-ts, etc., up to tM-tn-z, since it is apparent that in the interval t4-tn4, the curve of frequency variation of the Table Freq. of Voltage Harm. at Harms. Neer Freq. of Rm. (1,6m nem. (1,0m Hgh Hammam ma Kc.) Kc.) (Kc Reson. (Kc.) am 52a By examining the above table it can be seen that from time to to time tz, the voltage on the load resistor 52a varies from maximum when the 217th harmonic is in resonance with the tuned network comprising elements 31a, 38a, 39a and 40o; through minimum when the harmonics nearest the resonance frequency of the tuned network are out of resonance, and back to maximum when the 216th harmonic is in resonance with the tuned network. This represents one complete cycle of a wave function having a period equal to the time intervalfromtotos.

wave-functions developed across the diode-load- 66 resistor 52a is downward, until the interval Returning to the table it can be seen that the frequency differentiation of the oscillator Iza output for the interval tM-tn-z, is .3205 kilocycle and since the frequency variation during #30,000 second is 14.7465 kllocycles the time interval represented by tua-tan is .3205/14.7465 X .00005 equals .00000108665 second. The wavefunction occurring during the .interval tua-tn.:

l l would, if it were recurrent, have a frequency of4 920.2 kilocycles.

In a similar manner, the frequency differentiation of the oscillator |2a output for the interval tre-tn, shown in the table, is .3247 kllocycle and the timeinterval represented, therefore, is .3247/14.7465 .00005 equals .0000011005 second.

This wave-function would, if it were recurrent, have a frequency of 908.6 kilocycles, and this is substantially near the lower limit frequency (900 kc.) of the wave A as shown in the disclosure.

The interval tft-tun, shown in the table is the same as the interval tn-z-t, for, while the period from tu to tn represents the rising function of the output of the unit 20ab, the period from t to tn+n represents its declining function. The frequencies occurring during the period tft-M+ are the same as those occurring during the period trl-tn, but occur in reverse order. Thus, the frequency variation from time to to time t is from.

1,808.3 kilocycles to 908.6 kilocycles, and the frequency variation from time t to time M+ is 908.6 kilocycles to 1,808.3 kilocycles.

This fundamental-wave, together with its harmonic-waves, will appear in the primary inductance-coil 43a of the unit lia and will be transferred to the secondary inductance-coil 44a of the amplifying-unit Mia. The unit 16a merely serves to amplify these waves and supply them ,to the input of the band-pass filter Ila.

The band-pass filter 51a is designedand adapted to so attenuate the harmonic-waves supplied to it as to render them negligible in value at the output of the said filter, whereas, the fundamental-Wave having frequency-limits between 900 kilocycles and 1800 kilocycles inclusive, will be passed with minimum attenuation and is impressed upon the control-grid of the pentodetube 48a of the amplitude-modulator I8a.

The fundamental-wave (carrier-wave) last above referred to may, for the sake of clarity of illustration, be assumed to have the basic characteristics schematically illustrated in Fig. 5, though it is impossible, as a practical matter, to illustrate the carrier-wave having the actual frequency-limits referred to.

The wave above referred to, which is applied i to the control-grid of the tube 48a of the amplitude-modulator I8a, results in the appearance in the plate-circuit of the said tube of a frequencymodulated wave which, however, is also amplitude-modulated by the signal applied to the suppressor-grid of the tube 48a from its half of the secondary induotance-coil of the signal-input unit l9ab. In this connection, it may be explained that thevoice or other intelligence which it is desired to transmit, is applied to the said unit l9ab, and thisv resultant signal-wave may be assumed to have the form shown in Fig. 6, though it is not practical to show a normal actual signalfrequency.

The wave appearing in the plate-circuit of the tube 48a of the amplitude-modulator |8a as above described, is the now-doubly-modulated wavevA before referred to, and may, for purposes of understanding the fundamentals here involved, be assumed to be basically like the doubly-modulated wave shown in Fig. '7. This Wave A is applied to the primary inductance-coil of the couplingtransformer 24a and is transferred thereby to the secondary inductance-coil of the said transformer and by the latter is applied to the transmitting-antenna Illab or, if desired, to a wiredcircuit. l

The wave A'as above described, is doubly modulated in the sense that it 'is both lfrequency-mo'dii-LA lated and amplitude-motlule`.ted,` though; as vwill hereinafter appear in connection withthe descrip'- tion of Fig. 4, .the wave A may be vdoubly frequency-modulated. Y THE EvoLU'rxoN or .i TYPICAL Wlivr, B A

A wave B suitable foruse inconjunction with the wave A before described,v mayhave, with re1- spect to its frequency-modulated characteristics', a band-width having-a lower frequency-limitxof 1,000 kilocycles andan-upper frequency-limithof 2,000 kilocycles. :1 Under the circumstances above outlinedfthe wave-generator I2b, (which is a self-blocking super-regenerative detector or oscillator) will be constructed and arranged to rproduce a fundamental-wave having a fundamental-frequency-of 50 kilocycles, together with the harmonic-frequencies thereof, which latter have pronounced values. This fundamental-frequency will, however, be frequency-modulated by the frequencymodulator 20ab, so as to result in recurrent groups of waves, each corresponding to the others'in both pattern and time-interval, and all of which groups occur between an upper frequency-limit of 50 kilocycles and a lower frequency-limit of 35.42 kilocycles.

rate of quenching or suppression of the wavegenerator 12b. The harmonic-frequencies of the aforesaid fundamental-frequency will also be frequency-modulated by the frequency-modulator 20ab.'

It is well to here note that the output of the frequency-modulator 2Mb, before its application to the Wave-generator I2b, is amplifiedby the amplifying-unit 2lab and the amplifying-unit Under the circumstances above described, the

frequency-modulator Zllab will, as before noted,y

have an output-frequency of 10 kilocycles, in order to jointly with the wave-generator i2b, contribute to the frequency-modulated characteristics of the wave B. The output of the frequency-modulator 20a?) is, as before pointed out, characterized by a logarithmic or saw-tooth wave-form having the time of its rise corresponding to the time of its decline. While the outputwave of the frequency-modulator 20ab is logarithmic in character, as just noted, nevertheless suchlogarithmic wave-form is kept as` near to linear rise and decline respectively, as isfeasible. The amplitude-limiting unit 23ab performs in connection with the evolution of the vwave B in the same manner as that described in connection with the evolution of the wave A.

Returning now to the output of the wave-generator |2b, attention is again called to the factl that the said output comprises recurrent groups of fundamental-waves and recurrent groups ofA the harmonic-frequencies thereof. It is to be noted that such groups of waves and the harmonic-frequencies thereof (all of which'have been frequency-modulated) are applied to the input of the high-pass filter I3b.

Under the particular circumstances now being considered, the characteristics of the high-pass filter I 3b will be such as to cause the maximum attenuation of all frequencies below about 8,500

kilocycles in value (inclusive of the aforesaidgroups of fundamental-waves) while causing the minimum attenuation of all of the 'frequencies' at and above about 8500 kilocycles. The'ne't result will be that at the output of the high-pass The resultant frequencies between the limits just referred to, represents theV illter lib. the frequencies at and above about 8.500 kilocycles in value will appear with suiiicient value to be employed in the subsequent steps of the system, whereas. the frequencies below about 8,500 kilocycles will have negligible values and will not affect the subsequent steps of the system.

As before noted, the amplifying-unit Mb has a resonant network which includes the elements 31h and 3th which, in conjunction with the resonant network (including the elementsllb and IIb) of the unit llb, will freely pass only the selected-frequency which, in this' instance. will be a relatively-narrow band rcentered at about 8.500 kilocycles. The frequency-band last referred to will. of course, have been amplified in the unit IIb. As is well known in the art and as ore noted, frequencies other than the selectedrequency of about 8,500 kilocycles will pass from the unit IIb to the unit IIb, but in such negllible amplitude as not to affect the following stages of the system.

With the particular relationship now being described, the group of frequency-modulated harmonic-frequencies extending between and including the 170th to the 240th will successively appear in the resonant network comprising the elements 31h and 38h of the unit IIb, and the elements 39h and b of the unit lib. As each of the aforesaid frequency-modulated harmonicfrequencies (seventy in number) successively appears in the resonant network just referredto, each will at some point in the cycle of its frequency-change come intoresonance (about 8,500 kilocycles) with the aforesaid resonant network,

and will, while in substantial resonance. cause an increment of energy to appear in the said resonant network. Owing to their frequencymodulated character, the time-intervals between the respective maximum values of the said increments of energy will vary. Thisvariation in time-intervals results from the fact that the different harmonic-frequencies of the aforesaid fundamental-wave, progressively vary arithmetically in time-interval. Thus, the time-interval between the 170th and the 17lst harmonics may for convenience of illustration, be considered to be equal to V170. in which case the relative timeinterval between the 1'71st and the 1'I2d harmonics will be smaller and equal to 1,511, and so on to the 239th and 240th harmonics, which will have a still smaller relative .time-interval of $39. It is this arithmetical progression or relationship which determines the intimate frequency-modulation characteristics of the wave B.

The radio-frequency components of each of the increments of energy lust above referred to will vary in amplitude and the said increments will occur in groups of seventy, which groups provide what is commonly known in the art as a waveenvelope.

The radio frequency-waves representing each of the aforesaid increments of energy are rectified in the diode-section of the tube lib, with the result that half of each of the said waves and half of the envelope thereof are eliminated. The remaining halves of the said waves are so filtered by the load-resistor 52h and its complemental condenser 53h that there appears in amplied form in the output of the triode-section of the tube lib, a new frequency-modulated fundamental-wave having frequency-limits respectively at 1,000 kilocycles and 2,000 kilocycles, together with a series of harmonic-wavesrepresenting harmonics'of all of the waves occurring between 14 the said limits of 1,000 kilocycles and 2,000 kilocycles inclusive.

It may be here shown, by example, through one cycle of operation, how the output of the unit "ab, in conjunction with the operation of blocking oscillator lfb and the tuned network comprisi ing the elements I'Ib, 3th, Nb and Nb which il tuned to a relatively narrow band centered on 8,500 kilocycles, as well as the rectifying network in which the elements llb and 40h are included. contribute to the creation of the new frequencymodulated wave B appearing across the diodeload-resistor B2b and its complemental condenser IIb.

It may be assumed, for purposes of illustration, that at time to, the amplitude of the output of unit "ab as represented by the bias developed through the cathode resistor paralleling tubes b of amplifying unit 22h and tube 38h of blocking oscillator I2b is Erl-0, and the output of oscillator I2b is a function having a fundamental frequency of 35.4166 kilocycles.

It may be further assumed that the frequency output of unit 20Gb is, as shown above, 10.0 kilocycles per second.

A further assumption, for purposes of illustration, is that the rising function of the output of the unit "ab, which equals X, and the declining function which equals 1-X.=X represent straight lines. This assumption is permissible since in practical cases the logarithmic rise and decline of a saw-tooth wave can be made to approach linearity suillciently to support the illustration.

A still further valid assumption is that the frequency variation of the output of oscillator I2b is linear with the rise and decline of the function of the unit 2Mb output, since the actual logarithmic variation of the frequency of the 12b oscillator output, with the variation of bias voltage across the tube 35h cathode resistor can.be made to approach linearity sufficiently to support the illustration.

The fundamental frequency of 35.4166 kilocycles, at the output of oscillator 12b, has a large number of higher order harmonic frequencies harmonically spaced in frequency, so that the second harmonic has a frequency of 70.8332 kilocycles, the third harmonic has a frequency of' 106.248 kilocycles, etc. It can be seen therefore, that the 240th harmonic has a frequency of, substantially, 8500 kilocycles, and is, therefore, in resonance with the tuned network comprising the elements 31h, 38h, 39h and Mb. In consequence, the voltage developed across the element 52h,

which, together with its complemental condenser kilocycles, and is therefore substantially out of resonance with the tuned network comprising elements 31h, "b, 39h and 40h. Anci` since the 239th harmonic has a frequency of 8,482.253 kilocycles it, too, is substantially out of resonance withl the tuned network comprising elements 31h, 38h, 39h and 40h. As a result thelvoltage developed across the diode-load-resistor 52h is at a minimum.

A1, time n, the amplitude of che output of unit 20ab is such as to develop a bias of Eri-.055 voltacross the tube 35h cathode resistor, and the outfrom time to to time t1, and a rise from time t1 to time t2. And the decline, together with the rise, constitute a wave function which can be mathematically resolved into a fundamental sine wave, plus a large number of sine-wave harmonics of the said fundamental sine wave.

It may be convenient to further inspect the successive values involved in this example by means of the following table in which the first column heading Time indicates the progress of time from to to tn, while the function of the output of unit 20ab is rising from zero to maximum, Iand front tn to tn-i-n, while the function is declining from maximum to zero again, this representing one cycle in the operation of the unit 20ab.

16 diode-load-resistor B2b during the time t'o-tz were recurrent, the frequency of that function would be 1,968 kilocycles. This frequency is substantially near one frequency limit (2000 kc.) of the wave B, as shown in this disclosure.

To determine the frequency of the voltage wave-function developed across the load-resistor 52h, during the time interval tz-t4 we have a frequency differentiation in the oscillator |25 output of .1494 kilocycle, and, since the frequency variation during 1/zmoosecond is 14.5834 kiiocycles the timekinterval represented by tz-ti is,

equals .0000005122 second.

It is seen, therefore, that if the wave-function developed by the voltage across the diode-loadresistor 52h during the time interval tz-t4 were recurrent, the frequency ofv the wave-function would be 1,952 kilocycles. The frequency of that wave-function during the interval tz-ti is therefore 16 kilocycles lower than the wave-function during the previous interval tir-t2.

It may not be necessary to show, individually,

the 70 successive voltage wave-functions apnearing across the diode-load-resistor 52h during the successive time intervals ti-ts, tis-is. etc.,

Table Effect on Freq. of Freq. of Voltage Fre Out ut Hann. at Harms. Near Time ggbj hat of (05e. l b Resol?. Resol? Hghat HamnNm Iacoa (Vous (Kc.) (8500 c.) (8500 c (Kc.) (Kc.) Sisto: 52h

E+.0. 35. 4166 136+. 025-..- 35. 4006 Eri-.0 35.5648 E,+. 075. 35. 6394 E+. 100 35. 7142 Eri-4. 55..-. .4186 Eri-4. 49. 5626 E+4. 67. 49. 7076 E+4. 49. 8535 Erl-4. 000 Erl-4.73.... 49. 8535 Eri-4. 49. 7076 Erl- 025 35. 4906 By examining the above table it can be seen that from time to to time tz, the voltage on the load resistor 52h varies from maximum when the 240th harmonic is in resonance with the tuned network comprising elements 31h, 38h, 39h and 40h, through minimum when the harmonics nearest the resonance frequency of the tuned network are out of resonance, and back to maximum when the 239th harmonic is in resonance with the tuned network. This represents one complete cycle of a wave function having a period equal to the time interval from to to tz.

Since, as has been noted, the frequency of the output of unit 20ab is 10.0 kilocycles per second, and the time of the rising function of the 20a!) output represents one-half cycle of operation, the time from to to t is i/zoooo second.

And, since the output of oscillator |2b is 35.4166 kilocycles at time to, and 50.0 kilocycles at time t, the variation in frequency in 1/mmo second is 14.5834 kilocycles.

lThe variation in frequency of the oscillator 12h output from time to to time t: is 35.5648 minus 35.4166 equals .1482 kilocycle and. since the fref quency variation during 1/zmo second is 14.5834

kilocycles the time interval represented by to-tz 1S .1482/14.5834X.00005 equals .00000050811 sec- I ond. l

- It canbe seen, therefore. that if the function up to tri-e-tn-z, since it is apparent that in the interval t4-t-4, the curve of frequency variation of the wave-functions developed across the diodeltoadwiesistor 62h is downward, until the interval n-in-z.

Returning to the table it can be seen that the frequency differentiation of the oscillator 12b output for the interval tiH-tn-a, is .289 kilocycle and since the frequency variation during 1/zmmo second is 14.5834 kilocycles the time interval represented` by tn-4-tn-z is .289/14.5834 .00005 equals .0000009905 second. The wave-function occurring during the interval tn-4-tn-2 would, if it were recurrent, have a frequency of 1,009 kilocycles.

In a similar manner, the frequency differentiation of the oscillator 12b output for the interval tba-tn, shown in the table, is .2924 kilocycle and the time interval represented, therefore, ls .2924/14.5834 .00005 equals .0000010025 second.

This wave-'function would, if it were recurrent, have a frequency of 997.5 kilocycles, and this is substantially near the lower limit. frequency (1000) of the wave B as shown in the disclosure.

The interval tintua shown in the table is the same as the interval tn-n-tn for, while the period from to to te represents the rising function of the output of the unit 2Mb, the period from t to tu represents its declining function. The freof the wave developed by the voltage across the 1l quencies occurring during the period til-tw are 17 I the same as those occurring during the period to-tfa but occur in reverse order. Thus, the frequency variation from time to to time t* is from 1968 kilocycles to, 997.5 kilocycles, and the frequency variation from time t to time tu is from 997.5 kilocycles to 1968 kilocycles. i

-` This fundamental-wave. together with its harmanic-wave, will appear in the primary inductance-coll b of the unit lib and will be transferred-to the secondary inductance-coil b of the amplifying-unit lib. The unit lib merely serves to amplify these waves and supply them to the input of the band-pass iilter Ilb.

The band-pass lter I'Ib in this particular instance will be designed and adapted to so attenuate the harmonic-waves supplied to it as to render them negligible in value at the output of the said filter, whereas, the fundamental-wave having frequency-limits between 1,000 kilocycles and 2,000 kilocycles inclusive, will be passed with minimum attenuation and is impressed upon the control-grid of the pentode-tube b of the amplitude-modulator lib.

'I'he wave above referred to, which is applied to the control-grid of the tube b of the amplitude-modulator IBb results in the appearance in the plate-circuit of the said tube of a frequencymodulated wave which, however, is also amplitude-modulated by the signal applied to the sup- Dresser-srld of the tube 4 8b from its half of the secondary induction-coil of the signal-input'unit |9ab. As before explained, the voice or other intelligence which it is desired to transmit is applied to the said unit I9ab.

The wave appearing in the plate-circuit of the g tube 8b of the amplitude-modulator |8b as above described, is the now-doubly-modulated wave B before referred to. This wave B is applied to the primary inductance-coil of the coupling-transformer 2lb and is transferred thereby to'the secondary inductance-coil of the said transformer and by the latter is applied to the transmittingantenna IUab or, if desired, to a wired-circuit.

'I'he wave B above described, is doubly modulated in the sense that it is both frequency-modulated and amplitude-modulated though, as will hereinafter appear in connection with the description of Fig. 4, the wave B may .be doubly frequency-modulated.

The intelligence applied to the suppressergrid of the tube 48h will be in phase-opposition with respect to the intelligence applied to the suppresser-grid of the tube 48a, since the respective suppresser-grids receive their energy from opposite halves of the secondary inductance-coil of the signal-input unit I9ab.

'um RELATIONSHIPS oF THE WAVES A AND B Preferably and as before noted, the respective amplitude-modulations of the waves A and B are in phase-opposition (180 apart), but in the broader aspects of the present problem direct phase-opposition is not required, though some appreciable Ydegree of phase-differentiation should bc present to make available the maximum advantages of the present system.

'I'he primary object of the phase-differentiation between the amplitude-modulation characteristics of the wave A and B as above described. is to so distort the intelligence being transmitted as to render its reception by unauthorized persons extremely difncult, and even if the waves were successfully picked up by unauthorized persons, the intelligence carried 18 thereby will be difficult to extract in intelligible form.

No feasible mode i5 known of clearly and proportionally illustrating a composite of both the frequency-modulation characteristics and the amplitude-modulation characteristics of the respective waves A and B above referred to, but it is believed that the foregoing explanations with respect to the amplitude-modulation characteristics will be adequate. With respect to the relative frequency-modulation characteristics of the waves A and B, reference may be had to Fig. 3 of the accompanying drawings.

In Fig. 3, three waves are illustrated but the wave C may for the present be ignored, inasmuch as it is concerned with the receiving system.

In brief, it will be noted that in Fig. 3 the relationships of the waves A and B are such that each thereof has both its upper frequency-limit and its lower frequencylimit differing from the similar limits of the other of the said waves. In other words, with respect to the frequencymodulation characteristics. of the waves A and B, the respective lower frequency-limits thereof do not coincide nor do the respective upper frequency-limits thereof coincide. This is the preferred relationship of the waves, thoughy the respective upper frequency-limits of the said waves may coincide, or the respective lower frequencylimits of the said waves may coincide, but in each such instance, the relationship must be such that at least at one of the said limits, the said waves do not coincide.

THE RECEIVING SYSTEM As is shown in Fig. 2, the receiving system includes the receiving-antenna Hab before referred to, which antenna receives both of the doubly-modulated waves A and B described in connection with the transmitting system.

In the receiving system there is generated for purposes as will hereinafter appear, a third wave which may be designated by the reference character C, which in this particular instance has only frequency-modulated characteristics.

Tm: APPARATUS FOR PRODUCING THE WAVE C To provide the singly-modulated (frequencymodulated) wave C, a wave-generating unit 54e is employed which is connected through a highpass filter 55e to an amplifying-unit 56e. The amplifying-unit 56C just referred to is, in turn, inductively coupled to a combination rectifyingand-amplifying unit 5'lc. The output of the unit 51c is coupled through a band-pass filter 58e to the input of an inductive-coupling unit 59C, one of the outputs of which latter is coupled to one vof the inputs of a beat-frequency-detecting or heterodyne-detecting unit 60bc. The other output of the said inductive-coupling unit 59e is connected to a beat-frequency-detecting or heterodyne-detecting unit lac.

The beat-frequency-detecting or heterodynedetecting unit 60bc above referred to, is inductively coupled to an amplifying-unit 62. In turn, the output of the amplifying-unit 62 is inductively coupled to a combined rectifying-andamplifying unit 63.

The output of the combined rectifying-andamplifying unit 63 is connected to the primary of an audio-frequency transformer B4 having its output, in turn, connected to the input of an audio-frequency amplifier 65. The output of the amplifier 05 is, in turn, connected to the primary of an audio-frequency output-transformer 66, from the secondary of which latter the intelllgence received by the receiving-apparatus may be tapped oi to a loud-speaker or other translating-device. y

Returning now to the beat-frequency-detecting or heterodyne-detecting unit 6 lac, it will be noted by reference to Fig, 2 that one of the inputs of the saidv unit 6Iac is connected to the ,output of an amplifying-unit 61, while the other input of the said unit lilac, is connected to one of the outputs of the inductive-coupling unit 58C.

The output of the beat-frequency-detectlng or heterodyne-detecting unit 8Iac isy inductively coupled to an amplitude-limiting unit 69. -In turn, the output of the amplitude-limiting' unit 68 is connected to the input of a. frequency-modulator 10, which latter has its output connected to an amplifying-unit 1I. The output of the amplifying-unit 1I just referred to is connected y to the input of an amplifying-unit 12, which latter has its output connected to the input of the wave-generating unit 54e before referred to.

The various units 54e to 12 inclusive, above referred to, will now be described in sufficient detail to enable their respective functionings to be understood.

'I'he receiving-antenna I lab is connected to the control-grid of a pentode electronic-tube generally designated by the reference character 13 forming a feature of the amplifying-unit 61. The said tube 13 includes also a screen-grid and a suppresser-grid, as well a's a plate, which latter is connected to a primary inductance-coil 14. The primary inductance-coil 14 just referred to cooperates with two secondary inductance-coils 15 and 16 f arming features of the beat-frequencydetecting or heterodyne-detecting u-nit 6lac. Each of the said secondary inductance-coils 15 and 16 has respectively connected across it one of two variable condensers 11 and 18. Each of the said condensers acts in conjunction with its complemental inductance-coil to provide a resonant-circuit. The two inductance-coils 15' and 16 are connected in series with the control-grid of a pentode electronic tube 19 forming a. feature of the unit 6lac.

The tuned circuit in which the secondary inductance-coil 15 and its complemental condenser 11 are included, is resonant to a frequency widely different from the resonant-frequency of the circuit in which the coil 16 and condenser 18 are included. Therefore, vthe primary inductancecoil 14 is so constructed and proportioned as to pass the two frequencies to which the circuits of the secondary coils 15 and 16 are respectively responsive.

The beat-frequency-detecting unit 6lac also includes a primary inductance-coil 89 cooperating with a secondary inductance-coil 82. The said secondary inductance-coil is connected in series with the cathode and suppresser-grid circuit of the tube 19. The primary inductance-coil 80 is connected to one of the outputs of the inductivecoupling unit 59e, for purposes' as will hereinafter appear. The plate of the tube 19 is connected 'to a. primary inductance-coil 83;

The primary inductance-coil 83 of the unit 6lac cooperates with two secondary inductancecoils 84 and 65 forming features of the amplitudelimiting unit 69. Each of the said secondary inductance-coils 84 and 85 has respectively connected across it one of two variable condensers 86 and 81, each of which latter acts in conjunction with its complemental inductance-coil to provide a resonant-circuit.

The tuned circuit in which the secondary inductance-coil 84 and the condenser 86 are included, is resonant to a frequency widely different from the resonant frequency of the circuit in which the coil 85 and condenser 81 are included. The primary inductance-coil 83v is, therefore, constructed and arranged to pass lthe two frequencies to which the circuit of the secondary coils 84 and 85 are respectively responsive.

The circuit of the elements 84-86 of the unit 69 includes one of the plates of a twin diode electronic tube 88, while the circuit in which the elements 85 and 81 are located is connected to the other of the plates of the said tube.

It may here be noted that the resonant-circuit in which the elements 84-86 are included, develops a voltagel which is opposite to the voltage developed by the energy in the resonant-circuit of which the elements 85-81 form a part. These opposed voltages appear alternately at the output of the unit 69 and are applied to the grid of a gas-filled triode electronic tube 89 forming a feature of the frequency-modulator 18. f

In addition to the gas-filled triode 89, the frequency-modulator 10 also includes the usual resistors and a complemental variable condenser as shown, to thus produce an output-wave having saw-tooth or logarithmic characteristics. The frequency-modulator 10 also includes a condenser 89d and a push-button switch 89e, all in series with the cathode-plate circuit of the' tube 89.

As before noted, the output of the frequencymodulator 18 is connected through a variable `resistor and a variable condenser to the input of the amplifying-unit 1I. The amplifying-unit 1I includes a pentode electronic-tube 98 having its control-grid connected to the output of the frequency-modulator 10. The output of the amplifying-unit 1| is connected to the input of the amplifying-unit 12 before referred to.

The amplifying-unit 12 just above referred to includes a triode electronic-tube 9| having its cathode constituting the output of the amplifying-unit 12 and connected to the input of the wave-generator 54e before referred to.

The wave-generator 54e includes a triode electronic-tube generally designated by the reference character 92, which is here used as a super-regenerative detector or blocking-oscillator tube in a manner well understood in the art. The adjuncts of the tube 92 are schematically represented in Fig. 2 and do not require detailed description other than to mention that it is the cathode of the tube 92 which is connected to the output of the amplifying-unit 12. The platecircuit of the tube 92 is connected to the highpass filter 55e before referred to, which latter,

.in turn, is connected to the input of the amplifying-unit 56e.

The amplifying-unit 56e includes a pentode electronic-tube generally designated by the reference character 93, and having its control-grid circuit connected to the output of the high-pass filter 55e. Here'again, the adjuncts of the amplifying-unit 56e do not require detailed de scription other than to note that the plate-circuit of the tube 93 is made resonant by the inclusion of a condenser 94 anda primary inductance-coil 95. v l

The primary inductance-coil 95 of the amplifying-unit 56e cooperates with a secondary inductance-coi1u96 constituting the input feature ofthe combined rectifying-and-amplifying unit 51e. The coil 96 just referred to, in commotion with a condenser 91, provides a resonant-circuit 21 connected across the diode-section of a combined diode-triode electronic-tube generally designated by the reference character 00. 'I'he output of the resonant-circuit which includes the elements and 01. is connected to the grid of the triodesection of the tube 00, while the load-resistor 00 and associated condenser |00 of the diodesection. are connected to the cathode of the tube 00 through a biasing-battery |0I. The said biasing-battery serves to impose a bias-voltage on the grid of the triade-section of the combination tube 00. The plate-circuit of the triodesection of the tube 00 includes an yinductancecoil |02.

The inductance-coil |02 of the unit 51e constitutes the output element of the said unit 01e and is connected to the input of the band-pass mterilc before referred to. 'I'he output of the band-pass filter 50c is connected through a condenser |03/` to a primary inductance-coil |04.

Both the condenser |03 and the inductance-coil |00 may be considered to constitute features of the inductive-coupling unit 00o before referred to. The primary inductance-coil |04 cooperates with two secondary inductance-coils respectively designated by the reference characters |05 and |00 and also forming features of the unit 50c.

The said secondary inductance-coil |00 is connected to the primary inductance-coil 80 of the beat-frequency-detecting unit 0|ac, while the remaining secondary inductance-coil |05 of the unit 00e is connected to a primary inductancecoil |01 forming a. feature of the beat-frequencydetecting or heterodyne-detecting unit 60bc.

The primary inductance-coil |01 Just above referred to cooperates with a secondary inductance-coil |08 also forming a feature ofk the said unit 60bc. One terminal of the coil |08 is connected to ground through a bias-resistor |00 and a condenser H0. The other terminal of the secondary inductance-coil |08 is connected to the cathode of a pentode electronic tube III, which has its suppresser-grid-connected to the other or first-mentioned terminal of the secondary inductance-coil |08. The resonant plate-circuit of the tube includes a condenser 2 and\a primary inductance-coil |I3.

The primary inductance-coil I3` above referred to, constitutes the output-feature of the beatfrequency-detecting unit 60bc and cooperates with a secondary inductance-coil H4 constituting the input-feature of the amplifying unit 62. The said secondary inductance-coil H4, together with its complemental condenser H5, provides a resonant-circuit for the control-grid of a pentode electronic tube generally designated by the reference character H6, and forming a feature of the unit B2. The remaining adjuncts of the unit 62 do not require detailed description herein, other than to note that the output of the said unit includes a primary inductance-coil y I |1 and a complementa] condenser H0 connected to the plate of the tube H0 and providing a resonantcircuit for the output of the said tube.

The' primary inductance-coil H1 of the amplifying-unit 62 cooperates with a secondary inductance-coil H0 forming the input-feature of the combination rectifying-and-amplifying unit 02. The said secondary inductance-coil is complemented by a condenser |20 and together therewith provides a resonant-circuit for the diodesection of a diode-triode electronic tube generally designated by the reference character |2| also forming a feature of the unit 00. The controlgrid of the triade-section of the tube I2| obtains its bias in a manner well understood in the art from a suitably-connected battery |22, as is clearly indicated in Fig. 2. The plate of the triode-section of the diode-triade tube I2 l is connected to the primary inductance-coil of the audio-frequency transformer 04. The secondary inductance-coil of the transformer 0I is connected to the control-grid circuit of a Dentode electronic tube |22 of the audio-frequency ampliner 65 before referred to. The remaining ad- Juncts of the amplifier Il do not require detailed description herein, though it is to be noted that the plate of the said tube |22 is connected to the primary inductance-coil of the audio-frequency output-transformer l0. The terminals of the secondary inductance-coil of the transformer 06 may be connected to a loud-speaker or other suitable electrical translating-device.

In conjunction with scribed for producing the typical wave C, three batteries or other suitable sources of electrical energy |24, |20 and |20 are employed. In the particular instance shown, the battery |24 serves to supply the heaters of the various electronic tubes, while the battery |20 is suitably connected to such of the various electronic tubes as include a screen-grid, to impress a suitable intermediate voltage on such screen-grids. The battery`i25, 1n conjunction with the battery |20, is connected to the respective plates of the various electronic tubes with the exception of the twin diode plates of the tubes 00, 08 and |2|.

THE EvoLUrxoN or A TYPICAL WAVE C For purposes of clarity of description, let it be assumed that the frequency-modulated Wave C has a band-width having a lower-frequency limit of 1,500 kilocycles and an upper-frequency limit of 2,500 kilocycles.

Under the circumstances above outlined, the wave-generator 54o (which is a self-blocking super-regenerative detector or oscillator) will be constructed and arranged to produce a fundamental-wave having a fundamental-frequency of of 25 kilocycles, together with the harmonic-frequencies thereof. which latter have pronounced values. This fundamental-frequency will, however, be frequency-modulated by the frequencymodulator 'l0 so as to result in recurrent groups of waves, each corresponding to the others in both pattern and time-interval, and all of which groups occur between an upper frequency-limit of 25 kilocycles and a lower frequency-limit of 19.36 kilocycles. The resultant frequencies between the limits Just referred to, represent the rate of quenching or suppression of the wavegenerator c. The harmonic-frequencies of the aforesaid fundamental-frequency will also be frequency-modulated by the frequency-modulator 10.

Under the particular circumstances above described, the frequency-modulator 10 will have an output-frequency of 10 kilocycles, in order to contribute to, jointly with the wave-generator 54e, the frequency-modulated characteristics of the wave C. Furthermore, the output of the frequency-modulator 10 is characterized by a logarlthmic or saw-tooth wave-form (rather than a sinusoidal wave-form) having the time of its rise corresponding to the time of its decline, to thus correspond in this respect to the output of the frequency-modulator 20ab of the transmitting system (Fig. 1). As before noted in producing a logarithmic or saw-tooth wave-form, the said wave-form is kept as near to linear rise and de- 23 eline respectively as is feasible. Here again, such linear characteristics enable the present system to operate with maximum facility though they are not essential.

It is Well at this point to here note the function of the amplitude-limiting unit 69. This unit 69 serves to limit the amplitude of the output of the frequency-modulator in order to hold the frequency-limits of the wave C at predetermined values.

Before returning to the wave-generator 54C, it is also well at this point to call attention to the functioning of the amplifying-unit 61 and the beat-frequency-detecting or heterodyne-detecttecting unit 6|ac.

The amplifying-unit 61 serves to amplifyv the waves received by the receiving-antenna Hab and to amplify the same A(without discrimination) and supply tlie said waves in amplified form to the primary inductance-coil 14 of the amplifying-unit |51l for transfer tothe respectivesecondary inductance-coils 15 and 16 forming features of the beat-frequency-detecting un'it 6lac.

Briefly stated, the functionv of the beat-fre-` quency-detecting or heterodyne-detecting unit 6Iac is to cause the upper and lower frequencylimits of each cycle of frequency-modulation of each of the waves A,v Bv and C to coincide or be synchronized.

The series-connected secondary inductancecoils 15 and 16 of the unit 6lac are respectively tuned by means of their condensers 11-18, to the upper frequency-limit (1800 kilocycles) and the lower frequency-limit (900 kilocycles) of the wave A. It is to be explained in this connection, that while both the waves A and B are impressed upon the series-connected secondary inductancecoils 15 and 16, the respective resonant characteristics of the circuits of which they form a part are such as to respond only to narrow bands respectively just below the upper and just above the lower frequency-limits of the wave A. This arrangement is, as stated, for the reason that the wave A has arbitrarily been chosen asa control-wave, which functions in a manner as will hereinafter be described. Wave B, while rejected for the most part by the unit lilac, is utilized by the unit Bilbo in a manner as will also hereinafter appear.

The unit Blac rejects all of wave B except for the small part thereof, which comes into momentary resonance with that one of the resonantcircuits of the unit 6Iac which is tuned to about l 1,800 kilocycles.

83, what amounts in effect to two new waves appearing alternately and one having a frequency Let it .be assumed that the resonant network in which the elements 84 and 86 of the unit 6 8 are included is resonant to a frequency of '100 kilocycles, yand that the resonant network of which the elements 85 and 81 form parts, is resonant to a frequency of 600 kilocycles. The connections within the unit 69 are such that at one instant the circuit of one ofthe two diodeplates has'periodlcally impressed thereon a negative voltage, while the circuit of the other diodeplate has periodically impressed .thereon a positive voltage alternately with respect to the appearance of negative voltages in the circuit of the first-mentioned diode-plate.

The alternating negative and positive voltages above referred to are both impressed upon the control-grid of the gas-filled triode-tube 89 of the frequency-modulator 10 and provide the timing impulses for actuating the chargeand discharge-characteristics of the said tube 89.

As before noted, the frequency-modulator 16 includes a condenser 89d and a push-button switch 89e. These features are provided. to initially effect the synchronization of the waves A, B and C. This effect is achieved by closing the push-button switch 89e to cut in the condenser ,89dand thereby slightly change the frequency characteristics of the frequency-modulator 19. The retention of the push-button switch 89e in a closed position for a matter of a second or two will serve the purpose of effecting initial synchronization between the aforesaid waves A, B and C, and upon the release or -opening of the said push-button switch 89ev lthe inherent characteristics of the system will assert themselves and maintain the synchronization or coincidence of the said waves.

As before noted, the normal output of the frequency-modulator has a frequency of 10 kilocycles, which is applied in amplified values through the amplifying-units 1| and 12 to' the cathode circuit of the tube 92 of the wave-generating unit 54o.

Also as before noted, the output of the wavegenerating unit 54o will be a fundamental-wave having a fundamental-frequency of 25 kilocycles together with the harmonic-frequencies thereof.

substantially at 600 kilocycles and the other having a frequency substantially at 700 kilocycles.

In other words, the unit 6lac has mixed or heterodyned the characteristics of both the wave A andthe wave C to produce the-two alternatelyappearing waves above referred to, respectively having frequencies substantially at 600 kilocycles and 700 kilocycles.

The fundamental-frequency just referred to, .tof' gether with the harmonicsthereof, will be frequency-modulated by the frequency-modulator 10, so that the output ofthe wave-generating unit 54e is, in effect, recurrent groups of waves between an upper frequency-limit of 25 kilocycles and a lower frequency-limit of 19.36 kilocycles. l

Under the particular set of conditions here being'considered, the characteristics of the highpass filter 55e will be such as to cause the maximum attenuation of all frequencies below about 8325 kilocycles in value which is inclusive of the aforesaidV group of fundamental-waves appearing at the output of the wave-generating unit 54o. The characteristics of the said lter will also be such as to cause the minimum attenuation of all of the frequencies at and above about 8325 kilocycles. The net result will be that at the output of the high-pass filter 550, the frequencies at and above about 8325 kilocycles in value will appear with sufficient value to be employed in the subsequent steps of the receiving system, whereas frequencies below about 8325 kilocycles will have negligible values and willnot plied to the amplifying-unit 56e. amplianarco 25 tying-unit. as before noted, has a resonant network which includes the elements 94 and 9! which. in conjunction with the resonant network (including the elemen-ts 99 and 91) of the combined rectifying-and-amplifying unit 51e, will freely pass only the selected frequency which,

in this instance, will be a relatively-narrow band centered at Vabout 8325 kiloc'ycles. Here again, it may be noted, as is well known in the art, frequencies other than the selectedfrequencies of about 8325 kilocycles, will pass from the unit Btc to the unit 51c, -but in such negligiblev amplitude as not to affect the following stages of the receiving system.-

With the particular values now being' considered, the group of frequency-modulated harmonic-frequencies attending between and including the 333d to the 430th will successively appear in the resonant network comprising the elements 94 and 95 of thc unit 56o, and the elements 96 and 91 of the unit 51e. As each of the aforesaid vfrequency-modulated harmonic-frequencies (ninety-seven in number) successively appears in the resonant network just referred to, each will at some point in the cycle of its frequency-change come into resonance (about 8325 kilocycles) with the aforesaid resonant network, and will while in substantial resonance, cause an increment of energy to -appear in the said resonant network. Owing to their frequency-modullated character, the time-intervals between the respective maximum values of the said increments of energy will vary. This variation in time-intervals results from the fact that the different harmoule-frequencies of the aforesaid fundamentalwave progressively vary arithmetically in time-interval. Thus, the time-interval between the 333d and the 334th harmonics may for convenience of illustration be considered to be equal to 1/333, in which case the relative-time-interval between the 334th and the 335th harmonics will he smaller and equal to 1/334, and so on, to the 429th and 430th harmonics, which will have a still smaller relative time-interval of 1/429.' It is this arithmetical progression or relationship Awhich determines the ultimate frequency-modulation characteristics of the wave C. 1

The radio-frequency components of each of the increments of energy Just above referred to will vary in amplitude and the said increments will occur in groups of ninety-seven, which groups provide what is commonly known in the art as a wave-envelope.

The radio-frequency waves representing each of the aforesaid increments of energy, are rectifled by the diode-section of the tube 98 of the unit 51e, with the result that half of each of the said waves and half of the envelope thereof are eliminated. The remaining halves of the said waves are so filtered by the load resistor 99 and its complementa] condenser that there appears (amplified) in the output of the triode section of the tube 98, a new frequency-modulated fundamental-wave having frequency-limits respectively at 1500 kilocycles and 2500 kilocycles, together with a series of harmonic-waves representing harmonics of all of the waves occurring between the said limits of 1500 kilocycles and 2500 kilocycles inclusive.

It may be here shown, by example, through one cycle of operation, how the output of the unit 10, in conjunction with the operation of blocking oscillator 54e andthe tuned network comprising the elements 94, 95, 96 and 91 which is tuned to a relatively narrow band centered on 8325 kilo- 26 cycles, as well as the lrectifying network in which the elements 96 and 91 are included. contribute to the creation of the new frequency-modulated wave C appearing across the diode-load-resistor 99 and its compiemental Acondenser |00.

It may be assumed, for purposes of illustration, that at times to, the amplitude of the output of unit 10 as represented by the bias developed through the cathode resistor parellcling tubes 9i of amplifying unit 12 and tube 92 of blocking oscillator 54e is E+0, and the output of oscillator Mc is a function having a fundamental frequency of 19.3604 kilocycles,

It may be further assumed that the frequency output of unit 10 is, as shown above, 10.0 kilocycles per second.

A further assumption, for purposes of illustration, is that the rising function of the output of the unit 1li, which equals X, and the declining function which equals 1-X=X represent straight lines. This assumption is permissible since in practical cases the logarithmic rise and decline of a saw-tooth wave can be made to approach linearity suillciently to support the illustration.

A still further valid assumption is that the frequency variation of the output of oscillator 54o is linear with the rise and decline of the function of the unit 10 output, since the actual logarithmic variation of the frequency of the 54e oscillator output, with the variation of bias voltage across the tube 92 cathode resistor can be made to approach linearity suiiiciently to support the illustration.

The fundamental frequency of 19.3604 kilocycles, at the output of oscillator 54e, has a large number of higher order harmonic frequencies harmonically spaced in frequency, so that the second harmonic has a frequency of 38.7208 kilocycles, the third harmonic has a frequency of 58.0812 kilocycles, etc. It can be seen therefore, that the 430th harmonic has a frequency of, substantially, 8.325 kilocycles, and is, therefore, in resonance with the tuned network comprising the elements 94, 95, 96 and 91. In consequence, the voltage developed across the element 99, which, together with its complemental condenser |09, forms the load network of the diode rectifier of the tube 98, is at maximum.

At time ti, the amplitude of the output of unit 1n is Erl-.025 volt, and the output of blocking oscillator 54e has changed, as a result of the change in bias voltage, to 19.383 kilocycles. 'I'he frequency of the 430th harmonic is now 8,334.69 kilocycles, and is therefore substantially out of resonance with the tuned network comprising elements 94, 95, 96 and 91. And, since the 429th harmonic has a frequency of 8,315.3. kilocycles it, too, is substantially out of resonance with the tuned network, comprising elements 94, 95, 98 and 91. As a result the voltage developed across the diode load resistor 99 is at a minimum.

At time t2, the amplitude of the output of unit 10 is such as to develop a bias of Eri-'.05 volt across the tube 92 cathode resistor, and the output of blocking oscillator 54e is now 19.4055 kilocycles, as a result of this further change in bias. The 430th harmonic is now further out of resonance while the 429th harmonic has come into substantial resonance with the tuned network comprising the elements 94, 95, 96 and 91, and the voltage developed across the diode-load-resistor 99 is again at maximum.

It can be seen from the description above, that from time to to time t2 the voltage developed across the diode-load-resistor 99 has described a A, function which possesses a decline'for ifrom't'ime tototime ti, and arise -fromtimeftr ff'toftime tz. And thevdecline, together .with the yarise, constitute a wave functionv which Vcan vbev mathematically resolved into'a fundamental sine v y wave, plus a large number of sine wave harmonicsy .-3 of-the saidfundamentalv sine wave. I y

fIt may be' convenient to further inspect the *successivefvalues involved lin this example by 'means ofthe. following' table in which the first column heading .Time' indicates the progressy of time from to to tn, while the function of the out- 'vput of unit 10 is rising from zero to maximum,

he Aperiod and from tn to tu, while the function is declining from maximum'tozero again, this representing- 15 one cycle in the operation of the unit 1.0.

' successive time intervals t4-ts, ts--ta, etc., up to v 'developed by the voltage across the diode-loadresisto'r y99 during;` the time v, 'interval 221,.-'t4iwere recurrent,y the frequency -of'l the waveLfunction Vwould be 2,484.4 kilocycles.. The frequency of lthat wave-function during the interval tf-t4 is therefore 16.5 kilocycles'lower than the wavefunction during the previous 'interval to-tz.

It may not be necessary to show, individually, I

the 97 successive voltage wave-functions appearing across the diode-load-re'sistorv 99vl during the tn-4-tn a,'since it isfapparentthat-in the inten- `Table I Enea m94 Fredo: I vonage Fre Output Harm. at Harms. near req. o A Time Bgstbgo of sc.54c Reson. (8325 Reson. (8325 Hnat Harms. Near Maco (10 ts) (Kc.) Kc.) Kc.) (Kc.) Reson. (Kc.) Sister 99 `By examining the above table it can be seen that from to to time tz, the voltage on the load a wave function having a period equal tothe time interval from to to t2.

Since, as has been noted, the frequency of the output of unit 70 is 10.0 kilocycles per second, and the time of the rising function of the '70 output represents one-half cycle of operation, the time to to tn is 1/2o,ooo second.

And, since the output of oscillator 54o is 19.3604 kilocycles at time to, and 25.0 kilocycles at time t, the variation in frequency in 1/2o,uoo second is 5.6396'kilocycles. y.

'I'he variation in frequency of the oscillator 54e output from time to to time t2 is 19.4055 minus 19.3604 equals .0451 kilocycle and, since the frequency variation during 1/2o.ooo second is 5.6396 kilocycles the time interval represented by to-ta is .0451/5.6396 .00005 equals .00000039985 sec-V., ond.

It can be seen, therefore, that'if the function of the ywave developed by the voltage across the diode-load-resistor 99 during the time tc-tz were recurrent, the frequency o f that function would be 2,500.9 kilocycles. This frequency is substantially near one frequency limit (2500 kc.) of the wave C, as shown in this disclosure. I

To ldetermine the frequency of the voltage wave-function developed across the load-resistor 99,' during the time interval tz-t4 we have a frequencyl differentiation in the oscillator 5Ic output of .0454 kilocycle, and,` since the frequency variation during Vacuno second is 5.6396 kilocy- 35 wave-functions developed across the diodefl'oadresistor 99 is downward, until the intevervaltnq-- tn-il.

Returning to the table it can be seen thatthe frequency differentiationof the oscillator 54e 40 output for the interval tnq-tnz, is .0744 kilocycle and since the frequency variation'during 1/zo,ooo second is 5.6396 kilocycles the time interval represented by tn-4tnz is .0744/5.6.396 .00005 equals .00000065961 second. The vwavefunction occurring during the interval tn4-trn-z would, if it were recurrent.' have a frequency' of 1,516 kilocycles. y

In asimilar manner, the frequency differentiation of the oscillator 54e output for the interval tnz-tn, shown in the table, is .0749 kilocycle and the time interval represented, therefore, is .0749/5.6396 .00005 equals .00000066405'second- 'I'his wave-function would, if it were recurrent, have 'a frequency of 1,505.9 kilocycles, and this is substantially near the lower limit frequency (1500 kc.)4 of the wave C as shown in the disclosure.

'I'he interval trl-tum, shown in the table is the same as the'interval tn-z--tn, for', while the period from to to tn represents the rising function of the output of the unit 70, the period from tn to t`n+n represents its declining function.' The frequencies occurring during the period tntn+p are .the same as those occurring during' the period to-tn, but occur in reverse order. Thus, the frequency 'variation from time to to time tn is from 2500.9 kilocycles to 1505.9 kilocycles, and the frequency v variation from time tn to time tn+n is from 1505.9

lrepresented hete-is plied to it as to render them negligible in value at the output of the said filter, whereas, the fundamental-wave having frequency-limits between 1500 kilocycles and 2500 kilocycles inclusive, will pass with minimum attenuation and will be applied to the primary inductance-coll I of the inductive-,coupling unit lic.

The frequency-modulated fundamental-wave above referred to and having frequency-limits between 1500 kilocycles and 2500 kilocycles respectively. will be coincidentally impressed upon both of the secondary inductance-coils and l by the primary inductance-coil |04 of the unit 50c. The secondary inductance-coil |08 will apply this fundamental-wave to the primary inductance-coil Il of the unit Mac, while the seccndary inductance-coil |05 will through the connecting circuit, apply the said fundamental-waves to the-primary inductance-coil |01 of the beatfrequency-detecting or hcterodyne-detecting unit Ilbc. The effect of the aforesaid fundamentalwave within the unit llac has already been described, and the effect of the said wave within the unit be will now be described.

The band of frequencies represented by the wave C, namely, those between 1500 kilocycles and 2500 kilocycles inclusive, will be transferred by the primary inductance-coll |01 of the unit "bc to the secondary inductance-coil |08 thereof, and will be thereby impressed upon the cathode and suppressor-grid circuit of thetube III. Coincidentally, both the waves A and B will be impressed upon the. control-grid of the said tube which is connected to the receiving-antenna Ilab though the wave A will not be utilized, since, as before described, it is at this time being utilized in the unit Bloc. Therefore, as far as the unit 60bc is concerned, the wave' A need not be considered.

The band of frequencies between the respective limits of 1000 kilocycles to 2000 kilocycles, representing the wave B, are impressed upon the control-grid of the tube III, while the band of frequencies representing the wave C, namely, those between 1500 kilocycles and 2500 kilocycles inclusive, will be impressed upon the suppressorgrid of the tube Rectification of all of these frequencies will occur and there will be impressed upon the plate-circuit of the said tube III, for application to the circuit of the primary inductance-ooil III, what amounts, in effect, to a new amplitude-modulated wave (without frequencymodulation) having a frequency substantially at 500 kilocycles. 1

Stated briefly, it may be said that the beatfrequency-detecting or heterodyne-detecting unit be has mixed or heterodyned the characteristics of both the wave B and the wave C to produce a new relatively-narrow band of amplitude-modulad waves having substantially no frequency-modulation characteristics and centered substantially at 500 kilocycles.

Let it be assumed that the resonant network in which the elements H2 and H3 of the unit be are included, and the resonant network in which the elements Ill and ||5 of the unit 52 are included, is resonantto a frequency of about 500 kilocycles. This amplitude-modulated wave will be applied to the control-grid of the tube IIB of the amplifying-unit 52. It is 4to be understood in this connection, however, that the resonant network'in which the elements H2 to IIS inclusive are included is, as is well understood in the art, so characterized as to allow the passage modulations referred to.

The aforesaid amplitude-modulated wave of about 500-kilocyclefrequency, is applied to the plate-circuit of the tube ||6 of the amplifyingunit 52 and will be transferred by the primary inductance-coil' II1 to the twin diode-section of the diode-triade tube |2| of the combined rectifying-and-amplifying unit 63. The composite resonant network in which the elements H1, IllI Il! and I 20 are included, has resonance characteristics corresponding to those of the composite resonant network in which the elements Ill, H3, H4 and H5 are included. The aforesaid amplitude-modulated wave having a frequencyof about 500 kilocycles, will be rectied by the diodesection of the tube |2| of the unit 53, with the result that half of the said wave and half of the envelope thereof will be eliminated. The remaining half of the said wave is so filtered by the load-resistor which is in series with the diodesection of the tube |2| and its complemental condenser, that there appears on the control-grid of the triode-section of the .diode-triode tube I2 I, the reconstructed intelligence which has been applied to the input of the signal-input unit iab of the transmitting system of Fig.

The aforesaid reconstructed intelligence is applied to the primary inductance-coil of the audiofrequency transformer 5l and is transferred, of course, to the secondary inductance-coil thereof, and by the latter impressed upon the control-grid circuit of the tube |23 of the audio-frequency amplifier 65.4 The reconstructed intelligence in ampliiied values is now applied to the 'primary inductance-coil of. the audio-frequency outputtransformer 66. The said reconstructedv intelligence is transferred by the primary inductancecoil of the unit 66 to the secondary inductancecoil thereof, and may be taken off from the terminals of the said secondary inductance-coil to a loud-speaker or other suitable translating-device.

THE RELATIONSHIP OF THE WAVE C THE WAVES A AND B As before noted in connection with the doublymodulated waves A and B, the intelligence respectively imposed thereon is so distorted as to render its reception by unauthorized persons extremely diillcu-lt. This distortion is due to the phase-displacement existing between the respective amplitude-modulations impressed upon the said waves by the intelligence being transmitted.

In the receiving system of Fig. 2, both of the waves A and B are received, but the intelligence carried bythe Wave B alone will be reconstructed. The said doubly-modulated Wave B With the intelligence carried thereby (which latter is due to the amplitude-modulation), will be received by the unit Bobo in the manner of a, more or less conventional superheterodyne receiver and caused to heterodyne with the singly-modulated wave C The resultant wave is rectified as before described, and the intelligence brought in by the wave B is reconstructed and appears at the outlt of the audio-frequency output-transformer The intelligence arriving with the wave B is reconstructed under the control of the locallygenerated Wave C, which latter has evolved in turn, under the control of the wave A. The wave A, as before noted, has been diverted to the beattice. 

